Knowledge of cardiac output is crucial in the care of critically ill patients, as well as patients with chronic heart disease requiring monitoring of medication. For many years the standard of cardiac output measurement has been pulmonary artery catheterization. Previously known catheterization techniques, as described, for example, in U.S. Pat. Nos. 3,915,155, 3,726,269 and 3,651,318, involve periodic injection into the patient's bloodstream of a bolus of saline, during which thermodilution measurements are performed to determine cardiac output. Such techniques cannot generally be used for continuous monitoring. Moreover, such catheterization techniques pose significant risk to the patient, including malignant arrhythmias, pulmonary artery rupture, and in rare cases, death.
Consequently, for many years work has been underway to develop less invasive apparatus and methods for monitoring cardiac output. For example, as an alternative to catheterization methods, Doppler ultrasound techniques have been adapted to measure the velocity of blood flow. If the diameter of a vessel, its flow profile, and the angle of the ultrasound beam relative to the vessel can be determined, Doppler ultrasound measurements of the ascending aorta, either externally (from the suprasternal notch) or internally (from within the trachea) can be used as a measure of cardiac output.
U.S. Pat. No. 4,671,295 describes an example of such methods and apparatus, wherein an ultrasound transducer is mounted on the tip of an endotracheal tube so that Doppler measurements of blood flow from a point (pulse wave mode) or path (continuous wave mode) along the ultrasound beam can be measured. The method described in the patent requires multiple measurements within the blood vessel, a priori knowledge of the blood flow pattern and cross-sectional area of the vessel, and the relative angulation of the blood vessel. In addition, the measurement is highly dependent upon the exact placement of the transducer. These drawbacks have resulted in the slow adoption of Doppler ultrasound cardiac output techniques.
A yet further technique which the prior art has sought to apply to the measurement of cardiac output is bioelectrical impedance analysis ("BIA"). BIA has recently gained wide use as a method for measuring body composition and physiological metrics. BIA involves passing a low level electrical alternating current ("AC") through body tissues between multiple electrodes, measuring the voltage difference between multiple locations on the tissue, and then calculating the electrical impedance (electrical resistance plus reactance) of the stimulated tissue.
Generally, BIA apparatus employ two current electrodes to conduct a low level excitation current through body tissue. As current flows in the tissue, a potential difference develops across the tissue which is proportional to the value of the AC current and the tissue impedance. The tissue impedance may be calculated by disposing two sense electrodes between the current electrodes and measuring the voltage difference between the two sense electrodes.
Current flows predominantly through body materials with high conductivity, such as blood. Less current flows through muscle, which has an intermediate conductivity, while the conductivity of fat, air and bone is much lower than that of either blood or muscle. Because the resistance to current flow is a function of the conductivity and cross-sectional area of the conducting volume, volumes having a larger cross-sectional area have lower electrical resistance.
It is also known that the impedance of the conducting volume and the measured medium metrics (i.e., static parameters such as fat or water content, and dynamic metrics, such as blood flow) are dependent upon the placement of the electrodes and the conducting path between the electrodes. Thus, the greater the distance between the electrodes, the more likely that extraneous variables will affect the measurement.
Previously known BIA methods generally correlate the measured voltage drop between the sense electrodes to tissue impedance using relatively simple algorithms based on simplified models of body structure, for example, by assuming that the body is composed of simple cylindrical resistive volumes. Temporal cyclical variations in the body impedance are then assumed to result from physiological events such as blood flow and breathing.
Measurements of the electrical impedance, and particularly, the time-varying nature of electrical impedance, may therefore provide a non-invasive indicator of physiological events. Various algorithms have been developed to isolate specific physiological parameters, such as cardiac output, from the measured bioelectrical impedance, as described, for example, in W. G. Kubicek, et al., "Development And Evaluation Of An Impedance Cardiac Output System," Aerospace Medicine, Vol. 37, pp. 1208-1212 (1966) and U.S. Pat. No. 3,340,862, which is incorporated herein by reference.
Despite the application of BIA methods for measuring cardiac output, no simple continuous BIA-based cardiac output measurement device has gained widespread acceptance. Many existing BIA devices use external or internal electrodes to measure bioelectrical impedance for large volumes, for example, the whole body or thoracic segments. Because the excitation current diffuses throughout the entire volume, making use of any and all conductive paths, differences between individual patients, and even for the same patient over time, may inhibit standardizing the BIA metrics.
Moreover, it is known that while BIA measurements of cardiac output provide good correlation for normal patients and those hemodynamically stable patients, there is poorer correlation for critically ill patients and patients in heart failure, as described, for example, in R. J. Detemeter et al., "The Use Of Noninvasive Bioelectric Impedance To Determine Cardiac Output: Factors Affecting Its Accuracy," Am. J. Noninvasive Cardiol., Vol. 2, pp. 112-118 (1988).
An example of an attempt to overcome the variabilities encountered when taking bioelectrical impedance measurements across large volumes is described, for example, in U.S. Pat. No. 4,870,578. That patent describes BIA apparatus for monitoring cardiac output by using external electrodes that measure the electrical resistance of a segment of the thorax and includes circuitry to account for respiratory-induced voltage changes. As acknowledged in that patent, the respiratory-induced voltage changes are typically much greater than the cardiac-induced voltage changes.
Other devices that attempt to account for the effect of non-cardiac physiological events on bioelectrical impedance include arranging multiple electrodes on esophageal catheters to measure thoracic bioelectric impedance, as described, for example, in U.S. Pat. Nos. 4,852,580 and 4,836,214. Both patents describe multi-electrode arrays inserted into the esophagus to provide an impedance measurement reflecting blood flow in the descending aorta. It may be difficult for such devices to provide true isolation of cardiac-induced voltage changes from those induced by other physiological events. In addition, these systems do not ensure that the multiple electrodes make positive contact with the esophageal wall.
BIA measurements have also been employed to provide a metric of cardiac output by measuring physiologic effects other than blood flow. For example, U.S. Pat. No. 4,953,556 describes a BIA arrangement including an internal electrode mounted on an esophageal catheter and an external electrode which is disposed above the apex of the heart. The apparatus described in that patent attempts to use BIA measurements to determine cardiac wall motion and lung motion, from which an estimate of cardiac output and pulmonary activity can be obtained.
BIA measurements taken across small volumes, such as just the ascending aorta, are typically highly dependent on the position and orientation of the electrodes that are used to measure the impedance. For example, a pair of electrodes positioned orthogonally to the flow will provide radically different measurements than a pair of electrodes that are placed parallel to the direction of flow. Given the complex curvature of the aorta, it can be very difficult to align and orient a pair of electrodes to provide useful BIA measurements.
In view of the foregoing, it would be desirable to provide apparatus and methods for accurately, non-invasively and continuously measuring cardiac output using BIA techniques.
It further would be desirable to provide apparatus and methods for measuring cardiac output in critically ill patients using BIA techniques that overcome the inaccuracies arising from measuring voltage changes across whole body or large volume thoracic segments.
It also would be desirable to provide apparatus and methods for measuring cardiac output using BIA techniques that are less dependent on the precise positioning and orientation of the electrodes than previously known BIA cardiac output measurement devices and methods.
It also would be desirable to provide inexpensive apparatus and methods for measuring cardiac output using BIA techniques that overcome the drawbacks of previously known BIA cardiac output measurement devices and methods.
It would further be desirable to provide apparatus and methods for continuously monitoring cardiac output so as to permit the measured cardiac output to be employed as a metric for controlling and maintaining other aspects of a patient's health.